Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats

Setting the Valves.

that the copper hammer should always be used in these operations.)
Bolt down the bearing caps.

Fix the Valve Spindle Guide, valve spindle Stuffing Boxes, and
Valves, also the Governor Bracket with gear complete. Twist the
valve spindles round until the valve screw is placed symmetrically
with regard to the valve; then measure for equal play either side
of the guide bracket. In the case of the expansion spindle, put
in the intermediate rod, and let the lifting link be vertical when
valve is at half stroke. We shall proceed to set the valves; so
to aid us in turning the crank to its various positions, the flywheel
is driven on to the shaft, and there keyed. The governor pulley
can be put on afterwards, being in halves.

It is convenient to find the position of the main slide valve by
the aid of a thin wedge of wood, R, which is tried in the port on
the horizontal centre line, and on removal measured. Put the
main slide to open to 'lead' at the front of the cylinder, the
amount being known; place the crank horizontal, as taken from
the seatings, and put the crank pin to the front, as at s. Now
measure with a lath the length from valve spindle pin to nearest
edge of eccentric sheave.

Set the valve for lead to back of cylinder, place the crank in
horizontal backward position x, and measure the length as before.
The two lengths obtained should differ only by a very small
amount, and, being averaged, the length f the main eccentric
rod can be found. During the preceding operations the expansion
valve can be slid to one side or the other for convenience.

The expansion slide must be set centrally. We first move the
main slide to opening position at front and back part alternately,
and each time measure the distances u on the valve spindle. By
setting the spindle ta half the sum of the two measurements at u,
the valve will be central ^The expansion valve is now moved till
midway between the main valve ports (v v), and its spindle
measured as at u. Set the crank webs upright, as at w, with
|jf                   straightedge and plumblines. Take the distance, z, to' eccentric-

centre, found thus: >

z  radius of eccentric circle x sin /3;
and move tike expansion spindle back at u by Ms amount; then

;   .-     :         ,                 '                         .                '                                           *                                                                       XJ